Potential positive metal ions

Since different metals strip from mercury electrodes at characteristic peak potentials, several metal ions can be determined simultaneously. Metal ions which have been determined by ASV at a mercury electrode are BP, Cd, Cu, Ga, Ge, In, NP, Pb, Sb, Sn, Tl, and Zn. Solid electrodes such as graphite enable Hg, Au, Ag, and PP to be determined by ASV. In this case, the metal is preconcentrated on the surface of the electrode as a metallic film, which is then stripped off by the positive potential scan. 

In electrogravimetry [19], the analyte, mostly metal ions, is electrolytically deposited quantitatively onto the working electrode and is determined by the difference in the mass of the electrode before and after the electrolysis. A platinum electrode is usually used as a working electrode. The electrolysis is carried out by the con-trolled-potential or the controlled-current method. The change in the current-potential relation during the process of metal deposition is shown in Fig. 5.33. The curves in Fig. 5.33 differ from those in Fig. 5.31 in that the potentials at i=0 (closed circles) are equal to the equilibrium potential of the M +/M system at each instant. In order that the curves in Fig. 5.33 apply to the case of a platinum working electrode, the electrode surface must be covered with at least a monolayer of metal M. Then, if the potential of the electrode is kept more positive than the equilibrium potential, the metal (M) on the electrode is oxidized and is dissolved into solution. On the other hand, if the potential of the electrode is kept more negative than the equilibrium potential, the metal ion (Mn+) in the solution is reduced and is deposited on the electrode. 

If a positive potential is applied to the metal, as shown in Fig. 10.3, the ionization of the surface atoms will be promoted, and thus more metal ions will be produced at the surface. In the solution, water molecules, positive ions (cations), and negative ions (anions) drift around. The adsorbed layer of positive metal ions attracts nearby water dipoles in a preferential direction. The negative ions in the solution near the anode surface are also attracted toward the surface. The adsorbed fixed layer and the negative ion layer (Fig. 10.3) together are the so-called electrical double layer. Details about the double layer are available elsewhere [3]. Electrochemical reactions and mass transport for further electrochemical dissolution occur and pass through this double layer. 

Changing the potential, most often by making the metal more negative and thus eounteraeting the natural tendency of the positive metal ions to be transferred from the metal to the enviromnent... 

For metal synthesis, the polarization potential is usually negative as positive metal ions are reduced to zerovalent metals. This switch from the previous positive oxidation synthesis results... 

The common purine and pyrimidine nucleobases are shown in Figure 1. We will concentrate in this section on (N9)-substituted purines and (Nl)-substituted pyrimidines, the substituent being an alkyl group, and on the nucleosides which carry a (2 -deoxy)ribose residue at the correspOTding position. Metal ion binding of the free nucleobases is not of relevance in the present context. It is evident from Figure 1 that these nucleobase residues possess quite a number of potential metal ion-binding sites [30,58], yet from a narrow point of view one may say that N7 is the crucial site for purines and N3/(N3) for the pyrimidines. The details will be discussed below. 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

Inhibitors are materials that reduce either one or both of the partial corrosion reactions as in Fig. 2-5. Anodic or cathodic inhibitors inhibit the anodic or cathodic reaction respectively so that the rest potential becomes either more positive or more negative. Most inhibitors, however, inhibit the anodic partial reaction. This is because the transfer of metal ions can be more easily restricted than that of electrons. 

If the potential of the metal/solution interface is made more negative than p.z.c. by giving it an excess negative charge the positively charged ion will... 

Immunity the state of a metal whose corrosion rate is low or negligible because its potential is below (less positive than) that of equilibrium with a very small concentration (or activity of its dissolved ions. The metal is thus regarded as thermodynamically stable. Pourbaix has suggested that the small metal ion concentration be 10 mol dm (Atlas of Electrochemical Equilibria in Aqueous Solutions, p. 71, Pergamon/ CEBELCOR, Oxford (1966)). 

The question arises as to which metal is dissolved, and which one is deposited, when combined in an electrochemical cell. The electrochemical series indicates how easily a metal is oxidized or its ions are reduced, i.e., converted into positively charged ions or metal atoms respectively. The standard potential serves for the comparison of different metals. 

It is at the anode that oxidation takes place, with the anodic metal suffering a loss of negatively charged electrons. The resulting positively charged metal ions dissolve in the water electrolyte and metal wastage occurs. In the corrosion cell, the metal or metal area having the lowest electrical potential becomes the anode. 

As shown in Fig. 24, the mechanism of the instability is elucidated as follows At the portion where dissolution is accidentally accelerated and is accompanied by an increase in the concentration of dissolved metal ions, pit formation proceeds. If the specific adsorption is strong, the electric potential at the OHP of the recessed part decreases. Because of the local equilibrium of reaction, the fluctuation of the electrochemical potential must be kept at zero. As a result, the concentration component of the fluctuation must increase to compensate for the decrease in the potential component. This means that local dissolution is promoted more at the recessed portion. Thus these processes form a kind of positive feedback cycle. After several cycles, pits develop on the surface macroscopically through initial fluctuations. 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

---